MFJ 259B HF/VHF analyzer problem

Using the "DISTANCE TO FAULT" mode consistantly gives wrong readings, ie, the electrical length reads less than the measured length. Its like the velocity factor in reverse. Does anybody have any insights/experience with this problem?

I'm assuming that if this mode can't be trusted, then none of the others can be trusted either. I'm considering throwing it in the trash absent any solution/explanation.